Excimer gas lasers remain the principal laser system of choice for refractive eye surgery by photo-ablation, in which corneal eye tissue is vaporised while causing little or no thermal damage to adjacent areas. Notwithstanding their widespread use, excimer lasers have a number of inherent disadvantages, including large size and high operating and maintenance costs, and reliance on a gas that must be regularly replaced and is extremely toxic and therefore dangerous to ship and handle. Excimer lasers have an operating wavelength of 193 nm, in the ultraviolet region of the electromagnetic spectrum.
Alternative solid state laser systems have been proposed for generating an ultraviolet laser beam, suitable for corneal ablation, by frequency conversion of the output of an infra-red solid state laser, such as the widely used neodymium:YAG laser. The Nd:YAG laser produces a wavelength of 1064 nm, and this output beam is directed through a sequence of non-linear optical (NLO) crystals to derive an appropriate harmonic in the ultraviolet region by a process known as harmonic generation. Such systems are generally described in, eg U.S. Pat. Nos. 5,144,630 and 5,592,325. U.S. Pat. No. 6,381,255 discloses a solid state laser system in which an Nd:YAG laser beam is passed in sequence through a crystal of beta barium borate (β-BaB2O4 or BBO) and a pair of crystals of caesium lithium borate (CsLiB6O10 or CLBO) to generate the fifth harmonic of the Nd:YAG laser output at 213 nm, which has been found to be especially suitable for refractive surgery by photo-ablation. This harmonic has also been produced using three BBO crystals (Lago et, 1988, Optics Letters 13(3): 221-223).
For the harmonic generation process to work properly, the laser beam must pass through the non-linear crystal at exactly the right angle relative to the crystal structure. A very small error in the angle that the laser beam passes through the crystal can cause the conversion efficiency to drop significantly, possible even to zero. Fundamental problems arise from this sensitivity to angular presentation. Firstly, the exact required angle through the crystal usually depends on the temperature of the crystal and temperature gradients within the crystal. Secondly, the crystal usually absorbs a little of either or both the incident longer wavelength and the newly generated harmonic shorter wavelength. This absorbed laser energy heats the crystal, changing its temperature and creating temperature gradients within the crystal. Thus, the required angle through the crystal for efficient harmonic generation when the crystal is cold, ie. at the time the laser has just been switched on, is different from the required angle when the laser has been running for a while and its heating of the crystal has reached a steady state.
When a laser is first switched on and the laser beam passes through the crystal at the angle required for warm steady state efficient harmonic generation, it is not unusual for there to be no harmonic generation at all. In such an instance, the harmonic wavelength cannot contribute to heating of the crystal, and therefore the temperature state of the crystal that produces any harmonic generation is never reached. Even when the differences in angles between the cold starting condition and the warm steady state condition are not sufficient enough to create the problem described above, the changes in optimum angle do create long warm-up times and potentially large swings in the energy of the generated harmonic wavelength. To reach the fourth or fifth harmonic, for example 266 nm or 213 nm for an Nd:YAG system, the conversion process usually requires two or three crystal stages respectively. The instabilities of energy are thus multiplied for these shorter wavelengths.
Because of these difficulties, solid state wavelength laser sources have to date generally been considered unsuitable for industrial or medical applications.
The aforementioned U.S. Pat. No. 6,381,255 discloses arrangements for mounting the frequency conversion crystals in hermetically sealed housings with in-built heater elements for maintaining the crystals at optimum temperatures, which is important for the stability of the frequency conversion process. In one embodiment, the two CLBO crystals are mounted together in optical contact in the one housing, while in the other they are mounted in separate housings.
U.S. Pat. No. 6,381,255 also proposes keeping the laser pulse repetition rate low to allow the crystal to cool and partially return to its initial state between pulses. However, in many industrial applications the low pulse repetition rate makes the application uneconomic due to slow material processing rates. Even in the medical applications of laser refractive surgery, such a low pulse repetition rate can lead to impractically long treatment times. This is particularly true in the new types of treatments based on topography or wavefront linked customised ablations that require many smaller pulses to be applied to the cornea. Furthermore, with improvements in diode lasers in recent times there is now a preference that solid state lasers are diode laser pumped instead of flash-lamp pumped. Diode laser pumped solid state lasers are potentially more reliable and have better energy stability in their infra-red laser output than flash-lamp pumped solid state lasers. However, diode laser pumped systems are inefficient at the low pulse repetition rates proposed in U.S. Pat. No. 6,381,255.
Australian patent application 30076/89 proposes an arrangement of two or more optical crystals inside the laser resonator cavity. Each crystal has an individual temperature controller to adjust the temperature of the crystal for optimising performance. The orientation of each crystal is also adjusted to optimise performance.
To address small fluctuations in the direction of the laser beam as it emerges from the crystal, European patent publication 1 041 427 discloses a crystal holder fitted with “beam passage components” adjacent the incident and exit faces of the CLBO crystal to reduce localised air shimmer arising from the crystal heating system. In another arrangement disclosed in European patent publication 1 048 974, which is also concerned with reducing crystal interface degradation, elongate hermetically sealed spaces extend from the respective crystal housing windows, and these spaces are filled with high purity oxygen or a gas mixture of high purity oxygen and an inert gas.
International patent publication WO 02/33484 discloses an arrangement in which three OPO crystals are mounted within a common housing in respective holders that are rotatable for individual fine rotational adjustment of the crystals about respective axes. Individual Peltier heating elements are mounted in association with the respective holders for controlling the temperatures of the respective crystals.
U.S. Pat. No. 6,002,697 addresses the problem at hand by proposing mounting each of the three non-linear frequency conversion crystals in a separate sealed housing that is either purged with inert gas at a slight positive pressure or evacuated, to remove moisture from the housing and thereby prevent its contamination of the crystal. The temperature of each crystal is maintained at about “100° C. or more” by a heater in a feedback loop that includes a temperature sensor at the crystal. Tilt adjustment is by a set screw acting externally on the housing, which is rotationally supported.
Reference in the preceding discussion to particular patent documents or other publications is not to be taken as an admission that the disclosures therein are common general knowledge in Australia or elsewhere.
This invention is primarily directed to the provision of solid state laser systems of enhanced beam stability and uniformity, and with a longer crystal life than presently observed.